The present invention relates to a heating device for heat-sealing the bottom portions of containers for use with tubular blanks of rectangular to square cross section in folding a bottom forming end portion of each blank flat and heat-sealing the folded end portion.
Devices of the type mentioned are already known which have a hot air nozzle positionable as opposed to a bottom forming end portion of a tubular blank of rectangular to square cross section as fitted around an intermittently drivable mandrel for heating a region of the blank end portion, the nozzle being movable toward or away from the end portion on the axis of the blank. The hot air nozzle has a plurality of orifices directed toward a plurality of parts uniformly distributed over the entire region to be heated of the end portion when the nozzle is moved toward the blank end portion.
With the above conventional device, the state of the blank heated with hot air is dependent on the time during which the nozzle is opposed to the blank end portion; the longer the time, the greater is the degree of heating. While being advanced into the blank end portion, the nozzle is first opposed to a part of the blank near the extremity thereof and then to other parts progressively inwardly of the blank. Accordingly the time during which the nozzle is opposed to the blank is relatively longer at the part close to the blank extremity and is relatively short at the inward part. This results in the tendency for the former part to be heated to excess and the latter part to be heated insufficiently, entailing difficulty in uniformly heating the entire region to be heated.
In the event of excessive heating, polyethylene over the surface of the container blank disappears owing a high temperature and develops pinholes. Especially in the case where the blank includes aluminum foil, the foil becomes markedly inflated and is susceptible to formation of pinholes.
Furthermore, the blank is likely to become heated insufficiently and will not be bonded with high strength at the four corners of interior of the end portion or at the lapped portion of the container material having twice the thickness of the material because the material, which is in the form of a sheet, is made into the blank of rectangular to square cross section. If it is attempted to heat the insufficiently heated portion to a full extent, the excessively heated portion becomes heated more excessively.